1, Field of the Invention
This invention relates to a method and apparatus for manufacturing cold-rolled steel strip and more particularly to a method and apparatus for removing the scale formed on the surface of hot-rolled steel strip that is used as the breakdown in the cold reduction process.
2, Description of the Prior Art
In the cold reduction of hot-rolled coils, the scale formed on the surface of hot-rolled coils which serve as the starting material must be removed before they are subjected to cold reduction in order to obtain finished products of satisfactory surface quality. A popularly employed descaling practice is pickling by immersion in such acidic solutions as hydrochloric and sulphuric acids.
Ideas of continuously performing pickling and cold reduction have already been disclosed in Japanese Patent Publication No. 35594-1979, Japanese Provisional Patent Publication No. 127777-1981, and so on. Also already known is the direct linkage of cold reduction and continuous annealing processes that are carried out continuously. Still, a practice to continuously perform pickling, cold reduction and continuous annealing over a series of directly connected lines has been non-existent.
In order to perform continuous annealing, cold reduction and continuous annealing in succession using conventionally known means, a pickling tank according to Japanese Patent Publication No. 35594-1979, for example, must be placed upstream of a tandem cold reduction mill. But this combination presents the following problem.
Depending on the type of steel processed, the pickling rate of a coil can vary from one spot to another, such as in its leading end, middle and tail end. Accordingly, the leading and tail ends, which are usually slower to get pickled, cannot be pickled to the same extent as the middle portion unless they are passed through a pickling tank at a slower speed. The speed drop in the pickling tank entails a reduction in the threading speed on the following tandem rolling mill, which in turn unavoidably affects the strip travel speed through the following continuous annealing furnace. The speed change in the continuous annealing furnace has a direct bearing on the quality of the product. Besides, it is extremely difficult to keep a change in the annealing condition under good control. To allow the downstream processes to remain unaffected by such a change in the pickling rate, a long looper must be installed, with additional capital expenditure and operational complexity ensuing.
On the other hand, several methods have been proposed to perform descaling at low cost. A descaling method according to Japanese Provisional Patent Publication No. 89318-1981 comprises breaking the mill scale of hot coils on a four-high temper mill and subsequently pickling. A method according to Japanese Provisional Patent Publication No. 127835-1975 and Japanese Patent Publication No. 142710-1982 removes the mill scale that has been broken on a four-high temper mill with sweeping means and then subjects the stock to light pickling, liquid honing or other descaling treatment. Another method according to Japanese Provisional Patent Publication No. 209415-1983 pickles away the mill scale that has been broken by a tension-leveller-type scale breaker.
All these methods involve a step to mechanically break the scale on the surface, which is implemented by use of a four-high temper mill or a tension-leveller-type scale breaker. By providing a light draft or tension-induced elongation, the four-high temper mill and tension-leveller-type scale breaker initiate cracks in intrinsically brittle scale, eventually breaking it. The broken scale is removed from the hot coil surface in the next step.
Elongation given to the travelling hot coil results in the occurrence of cracks in, and the subsequent breaking of, the brittle mill scale formed thereon, running perpendicularly to the direction of elongation. Acid easily penetrates into the interface between the scale and base metal and also into the scale layer itself. So the cracked scale readily comes off from the metal surface on being pickled, or, otherwise, when it is mechanically brushed or shot-blasted.
But these conventional descaling methods are not without shortcomings.
Descalability depends on the chemical composition of the scale, the number of pores and cracks therein and the thickness thereof which, in turn, vary widely with the manufacturing conditions of the hot coil. Accordingly, metal surface damage due to overpickling or insufficient descaling due to underpickling could occur unless the hot coil is elongated to such an extent as will measure up to the descalability of the stock. If, for example, the amount of elongation is set to the material of the poorest descalability, steels or spots within the coil that are more sensitive to pickling might suffer from excessive attack of the base metal. Despite this, no attempt has been made to adjust the level of elongation to the descalability of the material. Under-elongation leads to the feeding of insufficiently descaled material coils into the subsequent cold reduction process and the impairment of the finished product surface quality. Over-elongation, on the other hand, necessitates a greater power requirement and electricity charge on the scale-breaking temper mill or tension-leveller-type scale breaker.